[Milsurplus] crystal has lost activity?

[john p hutchins]

Group -

Now educate me please.  A crystal is a rock,. that vibrates when 
electrical stress is applied through metal plates at particular angles 
to the crystalline grain?

I have seen many of these kinds of statements:
"After years it is possible that the  crystal has lost activity."

To stop a crystal form working would be corrosion forming on the metal 
plates or loss of conduction to the crystal surface. 

Any of combinations of these conditions could be rectified by cleaning, 
adjusting contact pressure on crystal surface, repairing conductive 
path?   So with the correct tools any crystal could be "fixed." to operate?

Am I off base?

Thanks
Hutch